In order to measure the capacity of a battery, they usually do this: they connect a resistor of a certain rating to this battery, which discharges this battery, and recording the values of the current flowing through the resistor and the voltage on it, wait until the battery is completely discharged. Based on the data obtained, a discharge graph is constructed, from which the capacity is determined. The only problem is that as the voltage across the battery decreases, the current through the resistor will also decrease, so the data will have to be integrated over time, so the accuracy of this method of measuring the battery capacity leaves much to be desired.
If the battery is discharged not through a resistor, but through a stable current source, this will determine the battery capacity with very high accuracy. But there is one problem. the voltage on the battery (18.104.22.168 V) is not enough for the operation of a stable current source. But this problem can be circumvented by adding an additional voltage source to the measurement circuit.
Fig. 1. Scheme for measuring battery capacity
V1. test battery; V2. auxiliary voltage source; PV1. voltmeter;
LM7805 and R1. stable current source; VD1. protective diode.
Figure 1 shows a schematic diagram of an installation for measuring battery capacity. Here you can see that the measured battery V1 is connected in series with the current source (it is formed by the integrated stabilizer LM7805 and resistor R1) and an auxiliary power source V2. Since V1 and V2 are connected in series, the sum of their voltages is sufficient for the current source to work. Since the minimum voltage required for the current source to work is 7 V (of which 5 V. is the voltage at the output of the LM7805 microcircuit, i.e. in this case it is the voltage drop across the resistor R1, and 2 V. this is the minimum allowable voltage drop between input and output LM7805), then for the operation of the current source, the sum of the voltages V1 and V2 is enough with some margin.
Instead of the LM7805 stabilizer, you can use another integrated stabilizer, for example, LM317 with an output voltage of 1.25 V and a minimum voltage drop of 3 V. Since the minimum operating voltage of the current source will be 4.25 V, the voltage of the second voltage source V2 can be reduced to 5 B. In the case of using the LM317 stabilizer, the stabilization current value will be determined by the formula I = 1.25 / R1
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Then, for a discharge current of 100 mA, the resistance value R1 should be approximately 12.5 ohms.
How to measure battery capacity
First, by selecting the resistor R1, you need to set the discharge current. usually, the value of the discharge current is chosen equal to the operating discharge current of the battery. It should also be borne in mind that some models of integrated voltage stabilizers 7805 may consume a small control current of the order of 2. 8 mA, so it is recommended to check the current value in the circuit with an ammeter. Next, a fully charged battery V1 is installed in the circuit, and having closed the switch SA1, the time starts counting until the voltage on the battery drops to the minimum value. for different types of rechargeable batteries, this value is different, for example, for nickel-cadmium (NiCd). 1.0 V, for nickel metal hydride (NiMH). 1.1 V, for lithium-ion (Li-ion). 2.5. 3 V, for each specific battery model, this data should be found in the relevant documentation.
After reaching the minimum voltage on the battery, switch SA1 is opened. It should be remembered that discharging the battery below the minimum voltage can damage it. Multiplying the value of the discharge current (in Amperes) by the discharge time (in hours), we obtain the battery capacity (Ah):
Consider the practical application of this method of measuring battery capacity using a specific example.
Battery Capacity Measurement NB-11L
Fig. 2. Li-ion battery NB-11L with a capacity of allegedly 750 mAh
Fits CAN.NB-11L 3.7V 750mAh
Use specified charger only
To connect the conductors to the battery terminals, two staples will be required, which should be bent as shown in Figure 3, and connected to "" and "-" battery terminals (Fig. 4.). It is necessary to avoid contact closure, it is better to insulate them.
Fig. 3. Home-made contacts for
connect to battery
Fig. 4. Home-made contacts,
connected to the battery
To measure the capacity of the NB-11L battery, its discharge current was taken equal to 100 mA. For this, the value of the resistor R1 was chosen a little more than 50 Ohms. The power dissipated on the resistor R1 is determined by the formula P = V 2 / R1, where V. voltage across resistor R1. In this case, P = 5 2/50 = 0.5 W. The LM7805 stabilizer should be installed on the radiator, but if there is no suitable radiator at hand, then the microcircuit can be partially immersed in a glass of cold water, but so that the leads remain dry (in the case of the TO-220 case).
After installing a fully charged NB-11L battery in the circuit and closing the SA1 switch, a countdown began with periodic voltage monitoring using the PV1 voltmeter. The data were entered in the table according to which the NB-11L battery discharge schedule was constructed (Fig. 5).
Fig. 5. Voltage graph on the NB-11L battery during its discharge with a current of 100 mA
This shows that after 5 hours of discharge with a current of 0.1 A, the voltage on the battery dropped to 3 volts and began to quickly fall further.
C = I t = 0.1 5 = 0.5 A = 500 mAh.
So the real capacity of the NB-11L battery was 1.5 times lower than that indicated on it.